1. Field of the Invention
The present invention relates generally to the measurement of magnetic fields, and in particular to the methods and apparatus for accurately detecting the presence of a weak scattered magnetic field in the presence of a known stronger field. In particular, embodiments of the current invention relate to methods for improved geophysical electromagnetic surveying.
2. Description of the Related Art
The removal of the effect of a known but unwanted magnetic field on a sensor is generally known as compensation, and is sometimes referred to as bucking. Compensation can be considered to have two distinct forms. In the first form, sometimes called active bucking, a first magnetic field is cancelled over a volume of space by creating a second magnetic field that is in opposition to it. In the second form of compensation, sometimes called passive bucking, the effect of a magnetic field detected by a sensor is cancelled by adding a voltage to the output of the sensor which is in opposition to the sensor's output.
There can be several reasons for wanting to remove a large magnetic field signal from a magnetic sensor. In particular, by removing a large part of the signal, thereby lowering the signal measured by the sensor, the effective dynamic range of the sensor can be extended, so allowing greater amplification and resolution of the field than would otherwise be possible. Additional reasons may include improved linearity and reduced slew-rate related noise. Furthermore, if compensation causes the magnetic field in the vicinity of the sensor to be reduced, there can be a corresponding reduction in noise caused by eddy current induction and induced magnetization in nearby metallic components.
For the purposes of this invention, a magnetic (H) sensor may be a magnetometer, as exemplified by a SQUID, a feedback coil, a fluxgate, an atomic vapour sensor, or similar device which is directly sensitive to the magnetic field, or a coil, a loop or similar electrical circuit element, which by virtue of Faraday's Law, is sensitive to time variations in magnetic flux density, or any instrument with similar functionality.
Compensation methods have found their way into a number of diverse applications, one of which is to suppress transmitted electromagnetic energy. For example, in document GB 2438057A to Robertson, electromagnetic radiation broadcast by a magnetic sensor is suppressed. In another example, Paschen et al disclose how to suppress transmission line noise in U.S. Pat. No. 5,920,130A. In a third example, Holmes and Scarzello use a set of three orthogonal Helmholtz coils to enclose an electrical device in U.S. Pat. No. 6,798,632 B1, also to suppress emitted power-frequency radiation.
Compensation methods can also be used to control magnetic field noise within a volume, as is common for rooms containing magnetic resonance imaging or electron beam devices. In such cases, currents sent through Helmholtz coils surround a volume to be shielded. Compensation is generally achieved by placing a magnetic sensor within the shielded volume, the signal from which is then used to generate a current in the coils and so annul the field at the sensor. This method is employed in document U.S. Pat. No. 5,465,012A to Dunnam, which uses three sets of orthogonal Helmholtz coils to compensate for a uniform magnetic field inside the coils, as does Kropp et al, in document US 2011/0144953, who consider the case of compensating for gradient fields. Buschbeck et al, in US document 2005/0195551, observe that in some applications involving particle beams, it is difficult to place the sensor in the volume where the field is to be annulled, and so two sensors, placed at two points, are used to interpolate the field value to be cancelled. Gelbien in U.S. Pat. No. 5,952,734 disclose an apparatus for maintaining a constant magnetic flux in a region by employing a coil energized by a servo loop and controlled by a flux lock circuit and a magnetic sensor. A compensation method which employed both coils and a magnetically shielded room was proposed by Buchannan in US document 2004/0006267. Wallauer in EP 2259081A1 proposed a magnetic field compensation method with a magnetoresistive sensor sensing the field within Helmholtz coils. Wallauer's invention split the incoming magnetic field signal into complementary high and low frequency components, with the low frequency component passed through an analogue to digital converter (ADC), a digital filter, then a digital to analogue converter (DAC) before being recombined with the high frequency component and passed to the Helmholtz coils.
Farjadad is US document 2011/0292977 discloses an ethernet based compensation circuit for well log applications in which a common mode signal is input to a controller to generate a compensation signal for application to a differential signal. The purpose of the invention is to pre-compensate the differential signal to reduce the effect or noise interference or imbalance in communication channels.
In the field of geophysical measurement, where the conductivity structure of the Earth is deduced from electromagnetic (EM) field measurements, compensation methods are common. A prevalent example of such compensation is found in active source electromagnetic prospecting systems. In an active EM system, a transmitter energizes a loop or coil with a periodic (steadily repeating) time-varying current. This current creates an electromagnetic field, typically referred to as the “primary” field, which energizes current flow within the Earth. These Earth currents create a “scattered” electromagnetic field which is detected by a receiver attached to the EM system. In many EM systems, the transmitter and the receiver are geometrically configured so that the primary field is orders of magnitude larger than the scattered field. In such cases, it is advantageous to employ compensation methods to remove as much as possible the primary field from the sensors allowing smaller scattered fields to be detected.
In many active source systems, compensation is implemented by achieving a balance between the primary field and a second field created by a bucking coil. In so doing, the net field from the two fields may be approximately annulled at the sensor.
Accurate balancing of the bucking with the primary field is best achieved when the coil geometries are fixed, as this also fixes the mutual inductances between the transmitter and the bucking coils, and their coupling to the volume where the fields are to be annulled. With the geometry of the coils fixed, accurate compensation at a single point may be achieved by placing the bucking coils in a series circuit with the transmitter coils and adjusting the moments of the respective coils so that the magnetic fields are in exact opposition. This approach works best in cases where the fields are not significantly disturbed by other sources of scattering, and where the coil geometry is rigid. It is particularly effective when the transmitter and bucking coils are in series and so have the same current waveforms, at least at frequencies well below those at which the coil capacitance influences the load impedance significantly.
An example of compensation is provided by Davydychev et al, who disclose an apparatus for adjusting the mutual inductance of a transmitter and receiver coil in US document 2010/0026280, with both a bucking coil and a trim coil. The trim coil is included to permit the field of the bucking coil to be adjustable, so improving the quality of the null that can be achieved. Another example is seen in the field of ground geophysical measurement, where Bosnar in US document 2009/0295391 A1 discloses an instrument for simultaneously measuring both the static magnetic field and the time-varying electromagnetic (EM) response of the ground. Bosnar uses a rigid geometry in which a Helmholtz-type compensation coil is used to annul the time-varying primary electromagnetic field at a magnetometer used to detect the Earth's static magnetic field.
For the reasons cited above, compensation is often required in airborne electromagnetic (AEM) measurements in which a controlled source transmitter loop is employed. An example of an AEM system employing compensation is provided in US document 2010/0052685 to Kuzmin and Morrison, which discloses a flexible AEM apparatus, commercialized as the VTEM AEM system. In the VTEM system, concentric transmitter and bucking coils are centred on a receiver. Bucking is also used in the Aerotem AEM system, in which a rigid geometry is employed, with compensation in the latter AEM system tending to be more effective than in the former because a rigid coil geometry used. The more stable bucking system of AeroTem versus VTEM is thus obtained at the cost of extra weight, implying a greater survey expense, and a large framework which is more expensive to ship and to repair if damaged. A means of accurately compensating a system with flexible geometry would be an advantage.
The primary field bucking just discussed permits the electromagnetic receiver to be operated at a larger gain than would otherwise be possible absent compensation, and accordingly permits the scattered fields of the Earth to be measured with greater sensitivity. Even so, compensation systems employed in the current state of the art in AEM methods only compensate the primary field of the transmitter. Yet there are other strong sources of magnetic field variation in various forms of noise which also degrade the quality of measurement and limit the gain of the receiver. These include the effect of magnetic sensor rotation in the static magnetic field of the Earth, radiated energy from power lines and cultural sources, and spheric noise. In cases where an EM system is mounted on a metallic vehicle, such as the GEOTEM AEM system, or where EM measurements are made proximate to a large conductor, such at sea, or in a mine in the presence of conductive and/or permeable ores and infrastructure, compensation that could dynamically respond to the changing conductive environment would be an advantage.
An additional effect which occurs in some AEM systems operating in the time domain occurs because the transmitter current waveform may take a finite time to propagate through the transmitter loop, an effect which may be noticeable at the receiver when the loop is rapidly energized with current. In such cases, the current in a compensation coil mounted in series with the transmitter coil may not be in-phase with the current(s) in the transmitter loop(s), so may require correction.
While bucking coils are intended to increase the quality of AEM survey data, these same coils may act as antennae and so may pickup and retransmit sources of background noise, creating an additional source of noise in AEM data set. Further noise could be caused by the change in coil coupling with respect to the static field of the Earth. Such considerations would not be a factor were the bucking field to oppose exactly the primary field at frequencies low enough that coil capacitances are not a factor. However, in practice exact cancellation is difficult to achieve and there may be a residual, uncancelled signal as a result, particularly in systems which are not rigid. It would therefore be advantageous to have a small, compact bucking system which could respond to such effects.
Furthermore, in an AEM system such as proposed by Polzer in document WO 2011/085462, where the receiver is on a motion-isolated platform, the receiver may translate or rotate with respect to the transmitter, so standard approaches to bucking which annul the primary field at a single point may be ineffective. In such cases, it would be advantageous to separate the compensation system from the transmitter loop and place it with the receiver. It is further advantageous in this case to create a digital bucking signal based on data sent to receiver module wirelessly rather than relying on an analogue series configuration. Such a configuration would be difficult to implement for this system as a direct electrical connection, as the direct connection would interfere with the motion isolation.